You will be happier with a bike that you build up yourself because everything will be exactly as you want it. However, it's almost always cheaper to buy one ready made.

If you are planning on commuting in a harsh environment, I'd recommend getting a dirt cheap used mountain or hybrid bike for the winter. Deicing chemicals will eat frames and components alive, no matter how well made so it's often wiser to treat your winter bike as disposable and save your 'good' bike for the summer.